The present invention relates to the field of semiconductor processing, and more particularly to a method and apparatus for polishing and/or planarizing semiconductor wafers and the thin films formed thereon.
Modern semiconductor devices are typically multilayered, having numerous metalization layers separated by numerous insulating oxides and interconnected with vias or contact holes. For instance, an interconnect for a typical multi-layer device is formed by depositing and patterning a first metal layer over the device, depositing an intermediate oxide over the patterned first metal layer, photolithographically defining a contact hole in the oxide, and depositing a second metal layer over the oxide that fills the contact hole and contacts the patterned first metal layer.
Patterning the first metal layer produces metal steps or undulations between where the first metal is removed and where the first metal remains. Because the intermediate oxide layer is a conformal layer, the oxide layer tracks these undulations. Accordingly, if the second metal layer were also deposited directly over the intermediate oxide layer, the undulations from the first metal layer would undesirably appear in the second metal layer.
Undulations in the second metal layer complicate patterning of the second metal layer, especially in high resolution, fine line-width applications, because no single focal plane exists on the second metal layer. A non-planar second metal layer, therefore, undesirably increases the line-widths produceable in the second metal layer. Furthermore, if the second metal layer undulations are large (e.g., on the order of the thickness of the second metal layer), voids or open circuits may form in the second metal layer. These problems may propagate to subsequently deposited material layers.
To prevent step or undulation propagation, the intermediate oxide layer is preferably planarized, removing any steps or undulations formed therein, prior to deposition of the second metal layer. Planarization is typically performed mechanically by forcing the semiconductor wafer face down against a semi-porous polishing pad which is saturated with an abrasive compound (i.e., a slurry) and by rotating the polishing pad relative to the wafer. The rotary motion between the polishing pad and the wafer mechanically removes layers of the intermediate oxide and is continued until the oxide steps or undulations are removed. This process is generally referred to as a chemical mechanical polishing (CMP).
To facilitate material removal during the CMP process the polishing pad is provided with grooves that channel slurry to the polishing pad/wafer interface, and that provide a path for wafer material to be removed from the polished wafer surface. During polishing, however, the downward force of the wafer against the polishing pad compacts slurry particles within these grooves, reducing the supply of fresh slurry to the polishing pad/wafer interface, the removal rate of wafer material, and the overall polishing efficiency and throughput of the CMP process, as well as giving rise to defects in the form of wafer scratches as described below. Additionally, the downward force of the wafer against the polishing pad causes the semi-porous surface of the polishing pad to pack down, causing polishing rates to become low and unpredictable, and necessitating frequent polishing pad replacement.
To extend the useful life of a polishing pad, a pad conditioner that roughens or xe2x80x9cconditionsxe2x80x9d the polishing pad surface is employed insitu, while the polishing pad polishes a wafer; or ex-situ, after wafer polishing is complete. A typical pad conditioner comprises a diamond surface that continually roughens the polishing pad surface by scribing additional xe2x80x9cmicrogroovesxe2x80x9d in the polishing pad surface. Continuous roughening of the polishing pad surface ensures adequate abrasion (e.g., due to slurry saturation of the roughened surface) at the polishing pad/wafer interface. (See, for example, U.S. Pat. No. 5,216,843 to Breivogel et al.).
While pad conditioners significantly increase a polishing pad""s abrasive lifetime, they do not address the problem of slurry debris (e.g., compacted, dried slurry) within the slurry grooves. In fact, during the polishing/conditioning process, the compacted slurry material which fills the pad""s original grooves maybe freed in large chunks that can scratch and produce defects in the polished wafer. Thus the polishing process itself can become a defect source.
Accordingly a need exists for a CMP apparatus and method that both extends the useful life of a polishing pad and eliminates wafer scratches caused by compacted slurry material.
The present invention addresses the shortcomings of the prior art by providing a chemical mechanical polishing (CMP) device that employs a brush for continually cleaning slurry particles from grooves (i.e., surface features in which slurry debris may collect), such as machined grooves, perforations or naturally occurring features. It will be understood that the pads described and claimed herein are hard pads such as those that are formed by casting (e.g., cast polyurethane), and that grooved pads refer to hard pads having surface features in which slurry debris may collect. The brush preferably comprises nylon bristles or other wear resistant material that is chemically stable in a corrosive CMP environment. The brush may be coupled in a stationary manner, or may rotate or roll, etc., as it impacts the polishing pad surface.
In a preferred embodiment the brush is coupled to a pad conditioner, such as a diamond embedded disk, and is scanned with the pad conditioner across the polishing pad surface. The brush may rotate with the pad conditioner if desired, or may be mounted to an anti-rotation device so as to remain stationary while the pad conditioner rotates. When coupled to the pad conditioner device, the brush is preferably spring loaded so that when the brush is not in. contact with the polishing pad, a polishing pad contacting surface of the brush projects beyond a polishing pad contacting surface of the pad conditioner. Thus, as the brush bristles wear they maintain sufficient contact with the bottom of each slurry groove to brush slurry particles therefrom.
Accordingly because the present invention continuously removes particles from the polishing pad grooves, no slurry debris builds up therein, and the present invention virtually eliminates defects caused by chunks of slurry debris such as particles that compact within, and subsequently dislodge from polishing pad slurry grooves scratching the wafer surfaces. A higher quality polished film results, scrapped wafer costs are reduced and thus the overall cost per wafer unit processed is reduced.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.